Smartphones need a button for "lock the screen but show what's currently on it and don't allow swiping through other things in my photo gallery or changing apps and don't dim the screen or show notifications or anything until I unlock it because I'm passing around a photo and some people have poor smartphone etiquette."

Everyone please read these thread before adding another response

The question has been answered several times!

Ninja Snap – Applications Android sur Google Play

Blague à vos amis qui aiment à regarder vos photos indésirables.

@bgcarlisle there was a thing for this in the Android 4.2 settings, but I forget what it was
@bgcarlisle What you're describing is a printout, and I'm not entirely opposed to this idea.
@bgcarlisle you mean like the iOS Guided Access feature?
@kenrachynski See the rest of the convo!
@bgcarlisle oh hey. I don’t think the rest of the convo showed up when I replied. Missed a refresh, I guess.
How to 'Pin' apps in Android 5.0 Lollipop

You can lock your device to a single app with a new feature in Lollipop called Screen Pinning.